DAK...Why do people ride smaller frames than years gone by

[pwa]

From an aesthetic point of view I prefer to see plenty of seatpost. It looks better to me. But at the front I don't think loads of spacers look great. Purely my own taste and nothing to do with function.

A sloping top tube and longer seatpost seem to me to present an opportunity to introduce some flex under the cyclist's bottom, but isolate that from the frame that has the job of keeping the front and rear wheels in a disciplined relationship. You can have the benefits of a stiff frame but have a bit of flex to save your rear. That appeals to me.

[Samuel D]

Frame stiffness is enhanced with a smaller frame

I can see why a shorter saddle-to-bottom-bracket distance may flex less, i.e. that taller riders would observe more flex.

What is not obvious to me is why a smaller frame would be stiffer if it’s ridden by the same person, i.e. has a necessarily longer seatpost.

Riddle me that, ye who may!

[foxyrider]

Frame stiffness is enhanced with a smaller frame

I can see why a shorter saddle-to-bottom-bracket distance may flex less, i.e. that taller riders would observe more flex.

What is not obvious to me is why a smaller frame would be stiffer if it’s ridden by the same person, i.e. has a necessarily longer seatpost.

Riddle me that, ye who may!

Not a riddle at all. It's quite simple really, the longer tubes allow a degree of twisting not seen in smaller frames. In turn (for the same material) this makes the energy loss through those tubes less. It can be offset by beefing up the tubes in larger frames but this isn't universally done and doesn't necessarily get the desired effect. Length of seatpost is something of a red herring as its the frame twisting not the seatpost and providing it has a suitable inserion length in the frame will not affect the frame dynamics.

There is a point where small is too small, you need to maintain the diamond shape for maximum strength. Frames with a zero or negative angle at the seatclamp are more likely to suffer upward cantilever flex and failure.

[pga]

Lighter and lower centre of gravity. We tend to ride with the saddle higher and the bars lower than in the 1950's after Hinault followed Cyril Guimard's advice in the 1970's.. Lab studies at the time endorsed this advice. This gave more power but less comfort, hence Hinault's subsequent knee troubles. The stages in major tours have reduced in length over the years hence the rational of this change Look at the old stage race pictures on YouTube to see how well into the 60's the frames were big with long wheelbases , the seat pillars often had only a couple of inches exposed and the riders sat well down. The long stages on terrible roads was the reason for this. In the 1970's as coaches we recommended that racing bikes had about four inches of exposed seat tube, just enough to grip with a hand. Everything worked back from this. At the same wheelbases were reduced. Many time triallists took this to extreme. Some may argue that touring cyclists have different needs. However in the 1970's and 1980's the classic Reynolds 531 tubed bikes that always won the big tours made excellent touring bikes. I have two Brian Rourkes with fairly short wheelbases, one built to race and one built to tour on. The frames are identical, the only difference being mudguard eyes on the tourer.

A bicycle can be a thing of beauty. How to define this is perhaps a matter of opinion. Why do Italian classic steel bikes often meet this criteria while French ones from the same period fail the test? 'The Rules: the Way of the Cycling Disciple' tries, somewhat tongue in cheek, to define what elements meet this criteria. Recommended reading for a rainy day.

[Brucey]

Frame stiffness is enhanced with a smaller frame

I can see why a shorter saddle-to-bottom-bracket distance may flex less, i.e. that taller riders would observe more flex.

What is not obvious to me is why a smaller frame would be stiffer if it’s ridden by the same person, i.e. has a necessarily longer seatpost.

Riddle me that, ye who may!

Much of the perceived frame stiffness is to do with how much the BB twists in relation to the handlebars when pedalling hard. This primarily comprises bending in the seat tube, and torsion in the down and top tubes. A shorter seat tube bends less, and in a smaller frame the top tube is closer to the centre of the torsion load, where it can do more good.

A seat tube that is 20% longer will (under cantilever conditions) deflect over 70% more; this is not a small effect!

However whether increased stiffness (over a certain point) makes you go faster or not in the course of a day's ride is a subject of debate.

FWIW one notion (which may like behind some older frame size preferences) is that if you are average height and you are going to have a frame built in PG tubes (or worse) then you may as well have the frame as big as you can manage; it will be a lot more comfortable this way. Anyone who has ridden (say) a 21" frame and a 23" frame back to back is likely to have noticed this sort of thing.

BTW there have been plenty of vague assertions about frame geometry in this thread, including that the top tube and the seat tube might be about the same length as one another in a traditional DF design. This can happen, but it is the exception. You are far more likely to find that the top tube barely increases in length with increases in frame size, especially with OTP frames. I recently examined the geometry charts for the Raleigh randonneur models from the 1980s and I found that (allowing for variations in seat angle too) the effective reach varied by something stupid like only 10mm from the 21" frame to the 25" frame. I can say with some confidence that I have never seen a 25" OTP frame with a top tube that is 4" longer than the 21" frame in the same range....

cheers

[pwa]

We normally consider a seatpost to be a component that inserts into the frame, but in a way it is an adjustable part of the frame, an extension of the seat tube. If you look at it that way, the seat tube/seatpost arrangement must be the same length for a particular rider no matter how you vary the proportions. What changes is amount below the junction of stays and tubes, within the "diamond", and the amount sticking up and not restrained except at the bottom. A long length of flexible seatpost must be able to flex more than a short length of flexible seatpost, and I'd be surprised if it did not flex more than a short seatpost, long seat tube arrangement, since less would be within the diamond. To make the most of that you would of course need a flexible seatpost.

[keyboardmonkey]

I'm 5'8". In 1987 I bought a Raleigh Road Ace with a 21.5" frame.

The bike is too big for me, really, but I so wanted the bike so got it. For years I wished that I had urged someone in the shop to get hold of a smaller model. However, a bit back I discovered that the 21.5" frame bike was the smallest that Raleigh made at the time.

Fast forward 25 years and I bought a Giant TCR Composite 46.5cm (small) frame. It is a much better fit - eventually I even got a slightly longer stem - so I for one prefer the options that modern manufacturing techniques (or is it marketing?) offer.

[Brucey]

....However, a bit back I discovered that the 21.5" frame bike was the smallest that Raleigh made at the time..

yes, and IIRC the effective reach is comparable to that of an average 23" frame of another type, i.e. ridiculously long for a 21.5" frame. Daft design....

cheers

[mattsccm]

Following on from the post above and Bruceys earlier one, I have 2 Raleigh Competitions. One a 21" and the other a 23 or maybe a bit bigger. Identical tope tube. The smaller was mine from new but now I find the bigger more comfortable as the reach is the same but the drip from seat to bars is less.
The small one was sized with some care, taking into account all sorts of body measurements. Still a good 2cm longer than my current bikes plus lower bars. I was 20 then and a keen time trialed which contributed I think. Too many aches to ride that low now.